A numerical visit of the relationship between hydraulic and mechanical apertures in natural granite fractures: Effects of heterogeneity and REV scale
Abstract
Fractures are critically important in subsurface systems, because fracture permeabilities could easily be orders of magnitude higher than the permeability of the overall or bulk porous rock matrix. Consequently, fluid, solute, and heat transport through fractured formations is typically controlled by advection through fractures of particularly high permeability, especially when they are arranged in parallel. Over the past several decades, considerable effort has been made towards understanding the characteristics of fluid, solute, and heat transport in single fractures and/or fracture networks. As a foundation, accurate analyses of the hydrogeology of fractured rock requires understanding the relationship between hydraulic and mechanical apertures, as apertures strongly control flow and transport characteristics. Numerous attempts have been made to relate the changes in mechanical apertures to those in hydraulic apertures. However, very few studies were able to present the direct relationship between hydraulic and mechanical apertures. In this study, we numerically investigate this relationship in natural granite fractures. We scan fracture surfaces both from tensile and shear fractures, using a photogrammetric scanning method. The two mated fracture surfaces are matched by means of image processing. Then, zero-stress mechanical apertures are derived for these mated fracture surfaces. Hydraulic apertures of representative elementary volumes (REVs) from sub-millimeter to centimeter scales are numerically determined through solutions of the full Navier-Stokes Equations in the presence of a fluid pressure gradient. The Navier-Stokes Equations are solved using a lattice-Boltzmann method with a lattice spacing of 5 mm. We find that the relationship between hydraulic and mechanical aperture is strongly dependent on the scale of the REV. Moreover, the anisotropy of hydraulic apertures becomes more apparent as the scale of the REV increases. We argue that channeling of fluid flow is responsible for our observations, because as the scale of REV increases, the majority of the fluid flow is dominated by preferential channels. Our findings have implications for the interpretation of laboratory fracture flow data and their upscaling to field and reservoir scales.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.H54C..03V
- Keywords:
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- 1822 Geomechanics;
- HYDROLOGYDE: 1847 Modeling;
- HYDROLOGYDE: 1859 Rocks: physical properties;
- HYDROLOGYDE: 5104 Fracture and flow;
- PHYSICAL PROPERTIES OF ROCKS